1. Technical Field
Devices for splitting a beam of light into multiple beams, and apparatus for measuring and testing of objects by detecting interference between the respective multiple beams and beams reflected back from one or more surfaces of an object.
2. Description of Related Art
In recent years, advances in optical fabrication technology have occurred that have enabled the production of increasingly complex non-rotationally symmetric optical elements. For example, in Proc. SPIE Vol. 3779, p. 434-444, Current Developments in Optical Design and Optical Engineering VIII, Robert E. Fischer, Warren J. Smith, Eds., “Capabilities of an Arch Element for Correcting Conformal Optical Domes,” Sparrold et al. disclose an arcuate-shaped non-rotationally symmetric optical element having a rotationally symmetric tip and a side characterized with astigmatic properties. In the related U.S. Pat. No. 6,310,730 of Knapp et al., the disclosure of which is incorporated herein by reference, there is disclosed an optical system including a curved window, an asymmetric, scoop-shaped optical corrector adjacent to a curved inner surface of the window, an optical train positioned such that the optical corrector lies between the curved window and the optical train, a movable optical train support upon which the optical train is mounted, and a sensor disposed to receive an optical ray passing sequentially through the window, the optical corrector, and the optical train. The optical corrector has an inner surface and an outer surface, at least one of which has a shape defined by an asymmetric polynomial.
As optical fabrication technology has improved, and the complexity of optical elements has increased, the need for improved metrology equipment to manufacture, measure, and inspect such elements has followed. For example, for the arch optic disclosed in the above reference of Sparrold et al., the two key metrology challenges are that the interior and exterior surfaces may not be spherical, and that the interior and exterior surfaces may not be parallel. There is a need to measure the location and orientation of surface regions on the optical element as well as the thickness of the optical element across a large range of points distributed over its working surface.
Interferometric methods for measuring the thickness and other physical or optical properties of an object are known. For example, U.S. Pat. No. 5,659,392 of Marcus et al., the disclosure of which is incorporated herein by reference, discloses an apparatus and method for measuring physical properties of an object, such as thickness, group index of refraction, and distance to a surface. The apparatus includes a low-coherence light interferometer and a coherent light interferometer in association so as to share a variable optical path delay element. Further disclosures of fiber optic interferometers and methods and apparatus using fiber optic interferometry are provided by Marcus et al. in U.S. Pat. Nos. 6,614,534, 6,038,027, 6,067,161, and 5,596,409, the disclosures of which are incorporated herein by reference. A metrology system that uses the principles disclosed in these patents is made and sold commercially as the OPTIGAUGE™ by Lumetrics, Inc. of West Henrietta, N.Y.
Although such apparatus and methods are effective for measurement of optical elements of certain shapes, they are not optimal for the measurement of complex non-rotationally symmetric optical elements.
What is needed is an optical profilometer that measures both the position and orientation of surface patches for general three-dimensional shapes. There is further needed an optical probe in such a profilometer that enables the measurement of the orientation of small regions on the surface of an optical element across an array of points distributed over the surface thereof, as well as the thickness of the optical element across the array of points on the surface thereof.